Opto-electrical devices incorporating metal nanowires

ABSTRACT

The present disclosure relates to OLED and PV devices including transparent electrodes that are formed of conductive nanostructures and methods of improving light out-coupling in OLED and input-coupling in PV devices.

BACKGROUND

1. Technical Field

This disclosure generally relates to opto-electrical devices such asorganic light-emitting diodes (OLEDs) and photovoltaic (PV) cells.

2. Description of the Related Art

An OLED emits light in response to an electric current. The lightgeneration mechanism is based on radiative recombination of excitons ofelectrically excited organic compound(s). FIG. 1 shows a conventionalOLED (10) formed on a substrate (12). An anode (14) is disposed on thesubstrate. The light-emitting layer takes the form of an organic stack(16) that includes a thin film of electroluminescent chemical compounds(18) flanked by two charge injection layers (20 and 22, one for electroninjection and one for hole injection). A cathode (24) is disposed on theorganic stack (16).

The cathode and anode provide the contacts for an external circuitry tosupply an electrical current, which in turn generates light (26) in theorganic stack (16). In general, in a bottom-emitting OLED, the anode(14) and the substrate (12) shall be transparent, allowing theinternally generated light (26) to exit from the substrate (12). In atop-emitting OLED, e.g., an OLED display, the cathode (24) shall betransparent, allowing the internally generated light (26) to exit fromthe cathode. The OLED may be entirely transparent, whereby both thecathode and the anode are transparent. A conventional transparentelectrode is indium tin oxide (ITO).

The internally generated light (26) propagates via various modes. Usinga bottom-emitting OLED as an example, FIG. 2 schematically shows certainpaths of the light propagation. It is desired that the light propagatesvia an external mode (30), i.e., by traveling through the transparentanode (14) and exiting from the substrate (12). However, not all thelight generated from the organic stack is capable of existing via theexternal mode. Instead, depending on the light incident angle on a giveninterface, a substantial portion of the light may propagate via a numberof waveguide modes. More specifically, at an interface where lightenters from a medium of higher refractive index to one of lesserrefractive index, a total internal reflection occurs at the interface ofthe two media for certain light incident angles, whereby no light exitsthrough the interface. As a result, a substantial amount of the lightgenerated from the organic stack is waveguided. One waveguide mode is acombined mode of the ITO layer (14) and the organic stack (16), both ofwhich have comparably high refractive indices. As shown in FIG. 2, light(32) is totally reflected at the interface (34) between the substrate(12) and the combined ITO-organic stack. Another mode is a substratemode, where the light (38, 40) is reflected at the interface (36) of thesubstrate and air. As shown, the waveguided light (38) may also exitfrom the edges of an OLED.

Thus, a critical parameter of OLED performance is the external coupling(“out-coupling”) efficiency, which is the ratio of photons emittedexternally over photons generated. There is a need for improving thelight out-coupling of OLEDs.

BRIEF SUMMARY

Described herein are opto-electrical devices with improved lightout-coupling for outputting light (e.g., OLED) or light input-couplingfor absorbing light (e.g., photovoltaic cells).

One embodiment provides an optical stack that comprises: a firstelectrode; an organic stack underlying the first electrode; ananostructure layer underlying the organic stack, the nanostructurelayer comprises a plurality of metal nanostructures; a high-index layerunderlying the nanostructure layer; and a substrate underlying thehigh-index layer, wherein the high-index layer has the same or a higherrefractive index than the organic layer, and wherein the nanostructurelayer forms a second electrode.

Another embodiment provides an optical stack that comprises: a firstelectrode; an organic stack underlying the first electrode; ananostructure layer underlying the organic stack, the nanostructurelayer comprises a plurality of metal nanostructures; and a substratesupporting the first electrode, organic stack and nanostructure layer,wherein an energy density of light that would be waveguided in theoptical stack without the nanostructure layer is reduced by inclusion ofthe nanostructure layer in the optical stack.

Another embodiment provides a top-emitting OLED that comprises: asubstrate, a first electrode disposed on the substrate; an organic stackdisposed on the first electrode; and a nanostructure layer including aplurality of nanostructures overlying the organic stack, wherein thenanostructure layer is a second electrode.

Further embodiments provide processes for fabricating OLEDs throughsolid-state transfer. One embodiment provides a process that comprises:providing a partial optical stack including a substrate, a cathodedisposed on the substrate and an organic stack disposed on the cathode,the partial optical stack having a top surface; providing a donor filmincluding a nanostructure layer on a transfer film, the nanostructurelayer including a plurality of nanostructures optionally dispersed in amatrix; and contacting the nanostructure layer of the donor film to thetop surface of the partial optical stack.

Another embodiment provides a process that comprises: providing a donorfilm by (i) depositing a plurality of nanostructures on a release liner;(ii) forming a matrix on the plurality of nanostructures, the matrixhaving a top surface; (iii) contacting a transfer film to the topsurface of the matrix, and (iv) removing the release liner to expose ananostructure surface; providing a partial optical stack including asubstrate, a cathode disposed on the substrate and an organic stackdisposed on the cathode, the partial optical stack having a top surface;and contacting the donor film by the nanostructure surface to the topsurface of the partial optical stack.

A further embodiment provides a process that comprises: providing adonor film by: (i) forming a matrix on a release liner, the matrixhaving a top surface; (ii) depositing a plurality of nanostructures onthe top surface of the matrix; (iii) reflowing the matrix to form areflowed matrix; (iv) pressing the nanostructures into the reflowedmatrix such that the transfer film contacts the top surface of thematrix; (v) removing the transfer film to expose the top surface;providing a partial optical stack including a substrate, a cathodedisposed on the substrate and an organic stack disposed on the cathode,the partial optical stack having a top surface; and contacting the topsurface of the donor film with the top surface of the partial opticalstack.

Yet another embodiment provides an OLED that comprises: a substrate; abottom electrode disposed on the substrate; an organic stack disposed onthe bottom electrode; and a metal film disposed on the organic stack,wherein the metal film has an outer surface and contacts the organicstack by a metal/organic interface, and wherein a first plurality ofnanostructures are disposed on the metal/organic interface.

A further embodiment provides an OLED that comprises: a transparentsubstrate; a transparent bottom electrode disposed on the transparentsubstrate; an organic stack disposed on the transparent bottomelectrode; and a metal film disposed on the organic stack, wherein themetal film contacts the organic stack by a metal/organic interface andhas an outer surface, and wherein a plurality of nanostructures aredisposed on the outer surface.

Yet another embodiment provides an OLED that comprises: a substratehaving a top surface and a bottom surface, the top surface being aninterface between the substrate and air; an anti-reflective layercontacting the bottom surface of the substrate; a first electrodedeposited on the anti-reflective layer, the first electrode comprising aplurality of conductive nanostructures; an organic stack deposited onthe first electrode, the organic stack comprising an organiclight-emitting material, a charge injection layer and a hole injectionlayer; and a second electrode deposited on the organic stack.

In a further embodiment, the substrate of the OLED has a firstrefractive index, and the organic stack has a second refractive index,and the anti-reflective layer has a third refractive index, and whereinthe third refractive index is larger than the first refractive index andless than the second refractive index.

In further embodiments, the anti-reflective layer of the OLED has anindex of reflection in the range of 1.5-1.8. In various embodiments, theanti-reflective layer is a polyimide layer.

Yet another embodiment provides a photovoltaic cell, which comprises: asubstrate having a top surface and a bottom surface, the top surfacebeing an interface between the substrate and air; an anti-reflectivelayer contacting the bottom surface of the substrate; a first electrodedeposited on the anti-reflective layer, the first electrode comprising aplurality of conductive nanostructures; a photo-active layer; and asecond electrode deposited on the photo-active layer.

In a further embodiment, the substrate of the photovoltaic cell has afirst refractive index, and the photo-active layer has a secondrefractive index, and the anti-reflective layer has a third refractiveindex, and wherein the third refractive index is larger than the firstrefractive index and less than the second refractive index.

In a further embodiment, the anti-reflective layer of the photovoltaiccell has an index of reflection in the range of 1.5-1.8. In variousembodiments, the anti-reflective layer is a polyimide layer.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, identical reference numbers identify similar elementsor acts. The sizes and relative positions of elements in the drawingsare not necessarily drawn to scale. For example, the shapes of variouselements and angles are not drawn to scale, and some of these elementsare arbitrarily enlarged and positioned to improve drawing legibility.Further, the particular shapes of the elements as drawn are not intendedto convey any information regarding the actual shape of the particularelements, and have been selected solely for ease of recognition in thedrawings.

FIG. 1 shows a prior art OLED.

FIG. 2 shows various waveguide modes in an OLED.

FIGS. 3(A)-3(B) shows a bottom-emitting OLED according to an embodimentof the present disclosure.

FIGS. 4(A)-4(B) shows a PV cell according to another embodiment of thepresent disclosure.

FIG. 5 shows a top-emitting OLED according to an embodiment of thepresent disclosure.

FIG. 6 shows a solid-state transfer process according to an embodimentof the present disclosure.

FIGS. 7-8 show alternative solid-state transfer processes according toembodiments of the present disclosure.

FIGS. 9(A)-9(D) show various embodiments directed to OLED configurationsthat reduce surface plasmon polaritons (SPP).

FIG. 10 shows waveguided light represented by its energy density withina device stack.

FIG. 11 shows the impact of a high-index layer for modifying the energydensity of the waveguide light.

FIGS. 12-15 show various embodiments in accordance with the disclosurefor incorporating a high-index layer in an optical stack device.

DETAILED DESCRIPTION

As disclosed herein, thin films of interconnecting conductivenanostructures (e.g., silver nanowires) are formed as one or moretransparent electrodes in an opto-electrical device such as an OLED or aPV cell. Unlike ITO, which can be costly to process, nanostructure-basedelectrodes can be advantageously fabricated in a solution-based approachor through a solid state transfer process from a donor layer, makingthem particularly suitable for large format or high throughputmanufacturing.

Nanostructure-Based Bottom Electrode

As light travels within a multi-layer device stack such as an OLED, theoptical behavior of the light can be influenced by at least one, or moretypically, all of the layers within the device stack. For instance, whenlight travels from a medium of a high refractive index to one of a lowerrefractive index, depending on the angle of the incident light, acertain degree of reflection may occur at the interface between the twomedia. In a bottom-emitting OLED, the internally generated light musttravel from the organic layer and the transparent bottom electrode, thenthrough the substrate in order to exit. Conventionally, because thecombined ITO/organic layer has a much higher refractive index (n₁≈1.8)than that of the glass substrate (n₂≈1.5), a substantial amount of thelight can be waveguided in the ITO/organic layer. Likewise, light thattravels from the substrate (n₂≈1.5) to the air (n₃≈1) may also bewaveguided in the substrate and the organic/ITO layer (see FIG. 2).

Various embodiments are directed to OLEDs in which conductivenanostructure-based transparent conductors are used in place of theconventional ITO layer. By simply eliminating the high-index-materialsuch as ITO, the distribution of the waveguide modes can be modified.However, the large difference between the indices of refraction of theglass and the organic layer will still cause losses due to totalinternal reflection at this interface. Thus, to further enhance lightout-coupling and reduce internal reflection, an anti-reflective layer isinterposed between the substrate and the bottom electrode formed by thenanostructure layer. Because the nanostructure layer is a network ofinterconnecting nanostructures (e.g., metal nanowires) rather than acontinuous layer, it does not completely inhibit the optical interactionbetween the organic layer and the anti-reflection layer.

FIG. 3A shows an OLED (50) according to an embodiment of the presentdisclosure. The OLED (50) includes a transparent substrate (54), ananti-reflective layer (58) interposed between the transparent substrate(54) and a nanostructure layer (62). The nanostructure layer comprises aplurality of metal nanostructures (66) and acts as an anode (i.e., thebottom electrode). An organic stack (70) is situated between the anodeand a cathode (76).

Optionally, an intermediate conductive layer (80) may be interposedbetween the organic stack (70) and the nanostructure layer (62). Theintermediate conductive layer is sufficiently conductive such that thecurrent is laterally distributed to achieve a uniform carrier injectioninto the organic stack (e.g., the light-emitting layer), therebyproviding uniform electroluminescence. The intermediate conductive layermay be, for example, a thin ITO layer, or a conductive polymer layer, ora layer of evenly distributed nanoparticles or nanowires. More detaileddescription of such a composite structure of the nanostructure layer(62) and the intermediate conductive layer (80) may be found in, forexample, U.S. Published Application No. 2008/02592, in the name ofCambrios Technologies, the assignee of the present disclosure.

In various embodiments, the index of refraction of the anti-reflectivelayer (n₄) should be a value between those of the substrate (n₂) and theorganic stack (n₅). More specifically, the index of refraction of theanti-reflective layer may be in the range of 1.5-1.8, or in the range of1.55-1.6, or in the range of 1.6-1.65, or in the ranges of 1.65-1.7, or1.7-1.75, or 1.75-1.8. For the most efficient reduction of internalreflection, the index of refraction (n₄) may be ascertained according tothe following formula:√{square root over (n₂×n₅)}

Thus, given the indices of refraction of glass (n₂≈1.5) and the organicstack (n₅≈1.8), respectively, the index of refraction (n₄) of theanti-reflective layer (58) should be 1.6 for this particularconfiguration. In a preferred embodiment, the anti-reflective layer is apolyimide layer.

The anti-reflective layer can be formed by direct deposition on thesubstrate. In forming the bottom electrode, conductive nanostructurescan be formulated into an ink composition (described in further detailbelow) and deposited directly on the anti-reflective layer. Such asolution-based approach enables large format and/or high throughputfabrication. Thereafter, the organic stack can be formed by knownmethods of the art. Because the organic stack is typically sensitive towater, prior to the formation of the organic stack, care should be takento ensure that the nanostructure layer is free of water, which is acommon solvent in the nanostructure ink composition.

Certain waveguiding may also occur within the anti-reflective layer (58)due to the reflection at its interface (84) with the transparentsubstrate (54). Thus, in another embodiment, as shown in FIG. 3B, theanti-reflective layer (58) may further contain a plurality oflight-scattering nanoparticles (88). The presence of thelight-scattering nanoparticles may force the waveguided or trapped lightin the anti-reflective layer out (also referred to as “extraction”).These light-scattering nanoparticles are also referred to as “scatteringcenters.”

In yet another embodiment, particle-based scattering centers may also beused in a PV cell in conjunction with a nanostructure-based electrode.An PV cell comprises a photo-active layer, which absorbs light andconverts it into an electrical current. In certain embodiments, thephoto-active layer is organic and the PV cells are also referred to asorganic photovoltaic (OPV) cells. Unless specified otherwise, theembodiments described herein are equally applicable to PV and OPV cells.

FIG. 4A shows an OPV cell (90) according to an embodiment. The OPV cell(90) includes a transparent substrate (94) and an overlyingnanostructure layer (102). The nanostructure layer comprises a pluralityof metal nanostructures (104) and acts as an electrode (e.g., anode). Aphoto-active layer (106) is situated between the nanostructure layer(102) and a cathode (108). Because the photo-active layer (106) and thenanostructure layer (102) have high indices of refraction of comparablevalues, they form a combined optical stack (107) in the propagation pathof light (110).

For an OPV device to work efficiently, the ratio of the absorbed photonsto the input photons need to be maximized. To maximize light absorption,the travel length or total travel time of the light (110) within thephoto-active layer should be as long as possible. In other words, it isdesirable for the light (110) to be waveguided within the combinedoptical stack (107). However, because it is not possible to createinternal reflection within the combined optical stack, waveguiding canonly be created by incorporating scattering centers therein.

FIG. 4B shows an embodiment for effectively creating waveguiding in anOPV device. In particular, a plurality of nanoparticles is incorporatedat the interface (109) of the combined optical stack (107) and thesubstrate (94). The nanoparticles, as scattering centers, facilitate thewaveguiding for light (111). In contrast to light (110) of FIG. 4A,light (111) of FIG. 4B is waveguided within the photo-active layer,thereby extending its total travel length and maximizing the lightabsorption.

The OPV device can be fabricated in a similar sequence as the OLEDdisclosed herein. In forming the electrode, conductive nanostructurescan be formulated into an ink composition (described in further detailbelow) and deposited directly on the substrate. The scattering centers(e.g., nanoparticles) may be formed by direct deposition on thesubstrate simultaneously with the nanostructures. Alternatively, thenanoparticles may be deposited prior to or subsequently to thedeposition of the nanostructures. Thereafter, the photo-active layer canbe formed by known methods of the art, followed by the formation of thetop electrode, which can be a metal plate.

Nanostructure-Based Top Electrode

Nanostructure layers are also suitable for replacing the ITO layer in aconventional top-emitting OLED.

FIG. 5 schematically shows a top-emitting OLED according to anembodiment of the present disclosure. The top-emitting OLED (112)includes a substrate (116), a cathode (120) disposed on the substrate(116), an organic stack (124) disposed on the cathode (120); and ananostructure layer (126) including a plurality of nanostructures (128).The nanostructure layer forms the transparent anode and top electrode ofthe OLED. The substrate and the cathode need not be transparent. Incertain embodiments, the cathode may be connected to a thin filmtransistor (TFT). It should also be recognized that the anode and thecathode could be exchanged or reversed.

It may be challenging to form a nanostructure layer as the topelectrode, due in part to the solvent sensitivity of the underlyingorganic stack. In particular, it is impracticable to deposit ananostructure ink composition directly on the organic stack because thesolvent, typically containing water, may significantly destabilize ordegrade the organic stack.

Thus, as an alternative to the solution-based approach, one embodimentprovides a method of forming a nanostructure layer on an organic stackthrough solid-state transfer process using a donor film. Morespecifically, a donor film is fabricated by pre-forming a nanostructurelayer a transfer film through a solution-phase deposition, and allowingthe solvent to fully evaporate. The nanostructure layer is thentransferred to the organic stack, thereby avoiding direct solventcontact with the organic stack.

As illustrated in FIG. 6, one embodiment provides a method comprising:

providing a partial optical stack (130) including a substrate (116), acathode (120) disposed on the substrate (116) and an organic stack (124)disposed on the cathode (120), the partial optical stack having a topsurface (134);

providing a donor film (136) including a nanostructure layer (126) on atransfer film (140), the nanostructure layer including a plurality ofnanostructures (128) optionally dispersed in a matrix (144); and

contacting the nanostructure layer (126) of the donor film (136) to thetop surface (134) of the partial optical stack (130).

In a further embodiment, the method further comprises removing thetransfer film (140).

In forming the donor film, an ink composition of nanostructures may bedeposited directly on the transfer film by known methods in the art,including slot die coating, spray coating, reverse offset printing, andthe like. The nanostructure layer forms after the volatile components ofthe ink composition are removed.

In another embodiment, the ink composition may further comprise a matrixmaterial (e.g., a binder). In this approach, the nanostructures and thebinder are co-deposited on the transfer film. Following deposition andmatrix curing, the nanostructures are dispersed in the matrix in asubstantially uniform manner, i.e., the nanostructures are distributedthroughout the entire thickness of the matrix. Although the matrix maybe conductive (e.g., conductive polymer) or non-conductive (e.g.,dielectric polymer), the nanostructure layer is conductive due to thepercolative conductivity between contacting nanostructures.

Another embodiment slightly alters the solid-state transfer approach byseparately depositing the nanostructures and the matrix, therebyallowing for more flexibility of the amount and the thickness of thematrix, while maintaining electrical contact between the nanostructuresand the organic stack. In addition, a release liner is employed toprovide a smooth surface of the conductive layer for contacting theorganic stack. More specifically and as illustrated in FIG. 7, themethod comprises:

providing a donor film (160) by

-   -   (i) depositing a plurality of nanostructures (128) on a release        liner (144);    -   (ii) forming a matrix (148) on the plurality of nanostructures        (128), the matrix having a top surface (150);    -   (iii) contacting a transfer film (152) to the top surface (150)        of the matrix (148), and    -   (iv) removing the release liner (144) to expose a nanostructure        surface (156);

providing a partial optical stack (130) including a substrate (116), acathode (120) disposed on the substrate (116) and an organic stack (124)disposed on the cathode (120), the partial optical stack having a topsurface (134);

contacting the donor film (160) by the nanostructure surface (156) tothe top surface (134) of the partial optical stack.

In a further embodiment, the method further comprises removing thetransfer film (152).

In yet another embodiment, the method further comprises first forming anintermediate conductive layer on the organic stack prior to contactingthe donor film to the organic stack. The intermediate conductive layeris preferably a continuous film such as a thin layer of ITO orconductive polymer. Such an intermediate conductive film can be helpfulto ensure that the contact between the nanostructures and the organicstack is uniform.

When prepared for deposition, the matrix may be combined with anappropriate solvent to assist with flowability, or deposited as neat ifit is a flowable material. Following the deposition, the matrix cures orhardens into a solid layer, either through removal of the solvent(s)and/or crosslinking. In certain embodiments, the matrix is athermoplastic polymer that, even after curing, may become reflowableupon heating or solvent infiltration, and hardened again upon cooling orsolvent evaporation. Generally speaking, crosslinked matrix may beformed through a photo-initiated or heat-initiated process and, oncehardened, is incapable of reflowing even upon further heating.

Advantageously, once the matrix is formed (cured and hardened), therelease liner can be removed, leaving a surface (156) of thenanostructure layer smooth and ready to make contact with the organicstack. Moreover, although the nanostructures are not necessarilydistributed throughout the entire height of the matrix; thenanostructure layer contacts the organic stack by the nanostructuresurface (156) to ensure maximum and uniform contact between thenanostructures and the organic stack.

Yet another embodiment provides an alternative approach to achieving asmooth contacting surface in the nanostructure layer. The methodinvolves first depositing a matrix on a release liner, followed bydepositing nanostructures. The nanostructures are not impacted into thematrix, but are deposited on the top surface of the matrix. The matrixmay be still flowable (prior to a fully cured state) or can be heated toa reflowed state. The reflowed matrix is in a semi-solid state andreadily deformable such that the nanostructures can be pressed into thematrix. More specifically and as illustrated in FIG. 8, the methodcomprises:

providing a donor film (160) by:

-   -   (i) forming a matrix (148) on a release liner (144), the matrix        having a top surface (150);    -   (ii) depositing a plurality of nanostructures (128) on the top        surface (150) of the matrix (148);    -   (iii) applying a transfer film (152) on the plurality of        nanostructures (128);    -   (iv) reflowing the matrix (148) to form a reflowed matrix;    -   (v) applying pressure to the transfer film (152) to press the        nanostructures (128) underlying the transfer film (152) into the        reflowed matrix such that the transfer film contacts the top        surface (150) of the matrix (148);    -   (vi) removing the transfer film (152) to expose the top surface        (150);

providing a partial optical stack (130) including a substrate (116), acathode (120) disposed on the substrate (116) and an organic stack (124)disposed on the cathode (120), the partial optical stack having a topsurface (134);

contacting the top surface (150) of the donor film (160) with the topsurface (134) of the partial optical stack (130).

In a further embodiment, the method further comprises removing therelease liner (144).

In an alternative embodiment, the method may omit the step of laminatingthe transfer film (152). Instead, the nanostructures 128 may becalendered with a calender roll directly into the reflowed matrix.

In yet another embodiment, the method further comprises first forming anintermediate conductive layer on the organic stack prior to contactingthe donor film to the organic stack. The intermediate conductive layeris preferably a continuous film such as a thin layer of ITO orconductive polymer. Such an intermediate conductive film can be helpfulto ensure that the contact between the nanostructures and the organicstack is uniform.

In the processes described above, it is possible that non-conductivecontaminating materials can be inadvertently introduced during thedepositions. Their presence on the nanostructure surface (156 in FIG. 7or 150 in FIG. 8) may inhibit the carrier injection/extraction.Accordingly, the method may further comprise, prior to contacting theorganic stack, surface treating the nanostructure surface of the donorfilm to minimize or eliminate any non-conductive contaminant. Thesurface treatment may involve Argon-plasma (or nitrogen-plasma) for abrief period of time. This surface treatment effectively removes thindeposition of contaminants on the nanostructure surface, therebyimproving carrier injection/extraction. Additionally, the surfacetreatment achieves a slight back-etching of the matrix, which provides abetter electric contact with the organic stack or the intermediateconductive layer.

The choice of the matrix is dictated by a number of factors. On oneside, the matrix has to provide some mechanical adhesion to the organicstack (e.g., pressure sensitive adhesive). On the other side, the matrixalso offers the ability to tailor the optical characteristics such asthe index of refraction, incorporation of scattering centers,incorporation of down-converter dyes, etc. The matrix can therefore beused to optimize the optical stack with respect to light extraction,emission uniformity and emission color. Thus, as disclosed in furtherdetail herein, in various embodiments, the matrix can be a crosslinkablepolymer or a reflowable polymer. The matrix may also be an optical clearadhesive. In addition, the matrix may also possess tailored opticalproperties (index of refraction, absorption, color etc.) and containparticles to impact the optical performance (e.g., scattering particles,etc.)

The matrix may also be patterned, e.g., by UV irradiation, prior totransfer. In this case, exposure to the UV irradiation in selected areasof the matrix causes crosslinking in the selected areas only, whereasthe non-exposed areas can be removed. The resulting pattern can then betransferred onto the organic stack. As an alternative to UV irradiation,LITI (Laser Induced Thermal Imaging) can also be used to locallytransfer the material onto the organic stack.

It should be understood that although the solid-phase transfer processis particularly suitable for forming a top electrode, it is not limitedto top electrodes. The process equally applies to forming bottomelectrodes. It should further be understood that, although OLED isillustrated, the processes disclosed herein apply equally to PV devices(e.g., OPV). That solid-phase transfer process ensures that theconductive network of nanostructures is exposed on one surface (156 inFIG. 7 or 150 in FIG. 8) that contacts the organic stack (e.g., alight-emitting layer in an OLED or a photo-active layer in a PV cell),thus providing carrier injection/extraction in OLED and PV devices.

Surface Plasmon Polariton Suppression

In a conventional bottom-emitting device, a solid metal cathode istypically used as the top electrode (see FIG. 1). At the interface ofthe metal and organic stack, energy losses may occur due to surfaceplasmon polaritons (SPP) or dipole interactions. Such energy lossdecreases the efficiency of the device. It has been shown that a higherroughness of the metal surface reduces the energy loss at thisinterface, see for example: Koo et al. Nature Photonics 4, 222 (2010) orAn et al. Optics Express 8 (5), p 4041 (2010).

Various embodiments provide devices in which nanostructures are placedon either or both sides of the metal electrode. The presence of thenanostructures increases surface roughness, thereby reducing SPP. Itshould be noted that it is not necessary to have a conductive network ofnanostructures. Rather, surface roughness may be sufficiently achievedby providing non-percolating nanostructures (e.g., nanowires or simplynanoparticles).

One embodiment provides placing nanostructures at the interface of themetal cathode and the organic stack. As illustrated in FIG. 9(A), anOLED (200 a) comprises a substrate (204 a), a bottom electrode (208 a)disposed on the transparent substrate, an organic stack (212 a) disposedon the transparent bottom electrode, and a metal film/cathode (216 a)disposed on the organic stack (212 a), wherein the metal film (216 a)contacts the organic stack (212 a) by a metal/organic interface (220 a),and wherein a plurality of nanostructures (224 a) are disposed on themetal/organic interface (220 a).

Because the metal cathode in an OLED is very thin (hundreds ofnanometers or less), SPP are also coupled at the outer surface of themetal cathode due to its close proximity of the metal/organic interface.Placing nanostructures at the outer surface of the metal film canperturb the propagation of SPP and lower the SPP losses in the OLEDdevice. Thus, as illustrated in FIG. 9(B), an OLED (200 b) comprises asubstrate (204 b), a bottom electrode (208 b) disposed on thetransparent substrate, an organic stack (212 b) disposed on thetransparent bottom electrode, and a metal film/cathode (216 b) disposedon the organic stack (212 b), wherein the metal film (216 b) contactsthe organic stack (212 b) by a metal/organic interface (220 b) and hasan outer surface (222 b), and wherein a plurality of nanostructures (228b) are disposed on the outer surface (222 b).

In a further embodiment, nanostructures may be placed on both sides ofthe metal film. As illustrated in FIG. 9(C), an OLED (200 c) comprises asubstrate (204 c), a bottom electrode (208 c) disposed on thetransparent substrate, an organic stack (212 c) disposed on thetransparent bottom electrode, and a metal film/cathode (216 c) disposedon the organic stack (212 c), wherein the metal film (216 c) contactsthe organic stack (212 c) by a metal/organic interface (220 a) and hasan outer surface (222 c), and wherein a first plurality ofnanostructures (224 c) are placed on the metal/organic interface (220c), and a second plurality of nanostructures (228 c) are disposed on theouter surface (222 c).

There are a number of approaches to deposit nanostructures on the outersurface of the metal film/cathode. In certain embodiments, since themetal film acts as a barrier of the organic stack, a solution-basedapproach to depositing nanostructures may be employed, including spincoating, slot-die coating, printing, and the like.

Alternatively, a transfer film may be used (as described herein). FIG.9(D) illustrates an OLED (200 d) comprises a substrate (204 d), a bottomelectrode (208 d) disposed on the transparent substrate, an organicstack (212 d) disposed on the transparent bottom electrode, and a metalfilm/cathode (216 d) disposed on the organic stack (212 d), wherein themetal film (216 d) contacts the organic stack (212 d) by a metal/organicinterface (220 d) and has an outer surface (222 d), and wherein aplurality of nanostructures (228 d) are disposed on the outer surface(222 d) and are embedded in a matrix (232 d). The nanostructures and thematrix have been previously formed on a transfer film (e.g., FIG. 7 or8), and transferred to the outer surface (222 d) of the OLED.

The matrix may be any of the matrix described herein. In theseembodiments (e.g., FIGS. 9A-9D), the optical properties of the matrixwill not play a major role because the OLEDs are bottom-emitting. Incertain embodiments, the matrix is opaque.

Placing nanostructures at the metal/organic interface, however, is notcompatible with the solution-based approach because of thesolvent-sensitivity of the organic stack. Thus, nanostructures may betransferred onto the metal/organic interface in a solid-state processbefore the metal film is deposited, e.g., by physical vapor deposition.

Positioning the Scattering Centers

In further embodiments, light out-coupling in an OLED device can befurther improved by maximizing the efficiencies of the scatteringcenters in the OLED device. As light travels through the device stack,it propagates in one or more modes. Scattering centers can bestrategically positioned to interfere with the behaviors of thepropagating light, especially those of the otherwise waveguided light.In particular, an energy density of light that would have beenwaveguided in an optical stack without the scattering centers can bereduced by inclusion of the scattering centers in the optical stack.However, for optical stacks that involve high refractive index layers(e.g., organic layer of an OLED or an organic photo-active layer of anOPV cell), waveguided light may have such limited amount of interactionwith the scattering centers that the scattering centers cannot beemployed efficiently.

FIG. 10 shows how waveguided light may not interact with the scatteringcenters in any appreciable way. In particular, FIG. 10 shows the energydensity distribution of the propagating light in a simplified devicestack (300). The device stack includes a first electrode (310), anorganic stack (320), and a glass substrate (360). Light generated by theorganic stack propagates within the device stack before it exits throughthe glass substrate. As described herein, the waveguide mode is mostlysupported within the organic stack due to the index difference betweenthe organic stack (320) and the substrate (360). As the energy densityof the waveguided light is represented by a bell curve (380), themaximum (390) of the bell curve is approximately centered within theorganic stack. Such waveguiding in the organic layer causes the light tohave little or a minimum interaction with any element beyond the organicstack (i.e., light scattering centers that underlie the organic stack,which are not shown for sake of clarity).

For the scattering centers to work more efficiently, they must be closerto (preferably at) the maximum intensity of the waveguided light withinthe organic stack. FIG. 11 shows how the energy density distributioncurve is drawn away from the center of the organic stack, therebyshifting closer to one end of the organic stack. More specifically, inan optical stack (400), a high-index layer (420) is disposed between theorganic stack (320) and the substrate (360). The high-index layer has acomparably high-index of refraction as that of the organic stack.Because of the optical continuity between the organic layer (320) andthe high-index layer (420), the energy density distribution curve (430)extends into said high-index layer (420). As a result, the maximum ofthe curve (430) shifts toward the high-index layer (420), resulting in agreater overlap (450) with the high-index layer.

Thus, in one embodiment, the energy density distribution can be modifiedin a way that its maximum is being shifted towards the location of thescattering particles. FIG. 12 is based on the concept of FIG. 11, andincorporates a plurality of nanostructures (340) between the organicstack (320) and the high-index layer (420). These nanostructures (e.g.,silver nanowires) function as the second electrode in an OLED device, aswell as scattering centers, which can facilitate extracting the lightgenerated and waveguided in the organic stack. The overlap (450) betweenthe scattering centers (340) and the energy density distribution curve(430) substantially increases as compared to the stack without thehigh-index layer (e.g., FIG. 10), thereby increasing the efficiency ofthe scattering centers for extracting the light. The overlap is definedas the under-curve area (in percentage) of the portion of the energydensity distribution curve that extends beyond the organic stack ascompared to the entire under-curve area of the curve. An example is area450 of FIG. 11. Typically, the larger the overlap, the more efficient itis for the scattering centers to extract the waveguided light. Invarious embodiments, the scattering centers are positioned in anoverlapped area that is at least 2%, or at least 3% or at least 5%, orat least 10%, or at least 30% or at least 50% to achieve the desiredresult.

As used herein, the “high-index” layer has a refractive index at leastthe same or more than the refractive index of the organic layer in whichthe mode is propagating. Typically, the high-index layer has arefractive index of 1.55 or higher, or preferably 1.6 or higher, or morepreferably, 1.7 or higher, or more preferably, 1.8 or higher.

Various embodiments describe different approaches to incorporating ahigh-index layer (420) that facilitate the efficiency of lightscattering centers (340). It should be understood that it is possible tointroduce several layers of scattering particles and its correspondinghigh-index layer to correspond to different locations of waveguidemodes.

FIG. 13 shows an embodiment according to the present disclosure. Adevice stack (500) includes a first electrode (310), an organic stack(320), a second electrode (510) having a plurality ofinterconnecting/networking nanostructures (340), a high-index layer(520) underlying the nanostructures (340) and a substrate (360). In thisembodiment, there is no insulating binder or matrix, and the secondelectrode is formed by directly depositing the nanostructures on thehigh-index layer. If processing concern requires that a binder materialbe present during coating of the nanostructures on the high-index layer,the binder may be subsequently removed (e.g., by washing or plasmatreatment) before forming the organic stack (320).

FIG. 14 shows a further embodiment according to the present disclosure.Device stack (700) combines the characteristics of device stacks (500)and (600). As shown, the device stack (700) includes a first electrode(310), an organic stack (320), a second electrode (710) having aplurality of interconnecting/networking nanostructures (340) embedded ina first high-index layer or matrix (720), a second underlying high-indexlayer (730), and a substrate (360).

FIG. 15 shows yet another embodiment according to the presentdisclosure. As shown, a device stack (800) includes a first electrode(310), an organic stack (320), a second electrode (810) having aplurality of interconnecting/networking nanostructures (340) embedded ina low-index matrix or binder 350, a high-index layer (820) underlyingthe second electrode, and a substrate (360). In this embodiment, thebinder is deposited with the nanostructures on the high-index layer(820) to form the electrode (810). The low-index binder then remains inthe electrode and should have a lower refractive index than the organicstack or the high-index layer.

In any of the embodiments described herein, the high-index layer canfurther include additional scattering centers, i.e., light scatteringparticles as defined herein.

In addition, the embodiments described in connection with waveguide modemodification can apply to top-emitting devices as well, in which casethe complete stack of the device (including the nanowire layer andhigh-index layer) are to be reversed.

The various components are described in more detail below.

Conductive Nanostructures

Generally speaking, the transparent conductors described herein are thinconductive films of conductive nanostructures. In the transparentconductor, one or more electrically conductive paths are establishedthrough continuous physical contacts among the nanostructures. Aconductive network of nanostructures is formed when sufficientnanostructures are present to reach an electrical percolation threshold.The electrical percolation threshold is therefore an important valueabove which long range connectivity can be achieved.

As used herein, “conductive nanostructures” or “nanostructures”generally refer to electrically conductive nano-sized structures, atleast one dimension of which is less than 500 nm, more preferably, lessthan 250 nm, 100 nm, 50 nm or 25 nm.

The nanostructures can be of any shape or geometry. In certainembodiments, the nanostructures are isotropically shaped (i.e., aspectratio=1). Typical isotropic nanostructures include nanoparticles. Inpreferred embodiments, the nanostructures are anisotropically shaped(i.e., aspect ratio≠1). As used herein, “aspect ratio” refers to theratio between the length and the width (or diameter) of thenanostructure. The anisotropic nanostructure typically has alongitudinal axis along its length. Exemplary anisotropic nanostructuresinclude nanowires and nanotubes, as defined herein.

The nanostructures can be solid or hollow. Solid nanostructures include,for example, nanoparticles and nanowires. “Nanowires” thus refers tosolid anisotropic nanostructures. Typically, each nanowire has an aspectratio (length:diameter) of greater than 10, preferably greater than 50,and more preferably greater than 100. Typically, the nanowires are morethan 500 nm, more than 1 μm, or more than 10 μm long.

Hollow nanostructures include, for example, nanotubes. Typically, thenanotube has an aspect ratio (length:diameter) of greater than 10,preferably greater than 50, and more preferably greater than 100.Typically, the nanotubes are more than 500 nm, more than 1 μm, or morethan 10 μm in length.

The nanostructures can be formed of any electrically conductivematerial. Most typically, the conductive material is metallic. Themetallic material can be an elemental metal (e.g., transition metals) ora metal compound (e.g., metal oxide). The metallic material can also bea bimetallic material or a metal alloy, which comprises two or moretypes of metal. Suitable metals include, but are not limited to, silver,gold, copper, nickel, gold-plated silver, platinum and palladium. Theconductive material can also be non-metallic, such as carbon or graphite(an allotrope of carbon).

Nanostructure Layer

In general, a nanostructure layer or coating acts as a transparentelectrode in the opto-electrical devices described herein. Thenanostructure layer (also referred to as a transparent conductor layer)is formed by depositing a liquid dispersion (or coating composition)comprising a liquid carrier and a plurality of conductivenanostructures, and allowing the liquid carrier to dry. Thenanostructure layer may also be first formed on a transfer film, thentransferred to an underlying layer in the opto-electrical device.

The nanostructure layer comprises nanostructures that are randomlydistributed and interconnect with one another. As the number of thenanostructures reaches the percolation threshold, the thin film iselectrically conductive. Other non-volatile components of the inkcomposition, including, for example, one or more binders, surfactantsand viscosity modifiers, may form part of the conductive film. Thus,unless specified otherwise, as used herein, “conductive film” refers toa nanostructure layer formed of networking and percolativenanostructures combined with any of the non-volatile components of theink composition, and may include, for example, one or more of thefollowing: a binder (e.g., a viscosity modifier), surfactant andcorrosion inhibitor.

The liquid carrier for the dispersion may be water, an alcohol, a ketoneor a combination thereof. Exemplary alcohols may include isopropanol(IPA), ethanol, diacetone alcohol (DAA) or a combination of IPA and DAA.Exemplary ketones may include methyl ethyl ketone (MEK) and methylpropyl ketone (MPK).

The surfactants serve to reduce aggregation of the nanostructures and/orthe light-scattering material. Representative examples of suitablesurfactants include fluorosurfactants such as ZONYL® surfactants,including ZONYL® FSN, ZONYL® FSO, ZONYL® FSA, ZONYL® FSH (DuPontChemicals, Wilmington, Del.), and NOVEC™ (3M, St. Paul, Minn.). Otherexemplary surfactants include non-ionic surfactants based on alkylphenolethoxylates. Preferred surfactants include, for example, octylphenolethoxylates such as TRITON™ (x100, x114, x45), and nonylphenolethoxylates such as TERGITOL™ (Dow Chemical Company, Midland Mich.).Further exemplary non-ionic surfactants include acetylenic-basedsurfactants such as DYNOL® (604, 607) (Air Products and Chemicals, Inc.,Allentown, Pa.) and n-dodecyl β-D-maltoside.

The viscosity modifier serves as a binder that immobilizes thenanostructures on a substrate. Examples of suitable viscosity modifiersinclude hydroxypropyl methylcellulose (HPMC), methyl cellulose, xanthangum, polyvinyl alcohol, carboxy methyl cellulose, and hydroxy ethylcellulose.

In particular embodiments, the weight ratio of the surfactant to theviscosity modifier in the coating solution is preferably in the range ofabout 80:1 to about 0.01:1; the weight ratio of the viscosity modifierto the conductive nanostructures is preferably in the range of about 5:1to about 0.000625:1; and the weight ratio of the conductivenanostructures to the surfactant is preferably in the range of about560:1 to about 5:1. The ratios of components of the coating solution maybe modified depending on the substrate and the method of applicationused. A preferred viscosity range for the coating solution is betweenabout 1 and 100 cP.

In one embodiment, the coating solution may initially contain a binder(e.g., HPMC) to facilitate film forming. However, the binder should beremoved thereafter such that the nanostructures form a discontinuouslayer and do not interfere with the optical interaction between theanti-reflective layer and the organic stack.

The electrical conductivity of the conductive film is often measured by“sheet resistance,” which is represented by Ohms/square (or “ohms/sq”).The sheet resistance is a function of at least the surface loadingdensity, the size/shapes of the nanostructures, and the intrinsicelectrical property of the nanostructure constituents. As used herein, athin film is considered conductive if it has a sheet resistance of nohigher than 10⁸ ohms/sq. Preferably, the sheet resistance is no higherthan 10⁴ ohms/sq, 3,000 ohms/sq, 1,000 ohms/sq or 350 ohms/sq, or 100ohms/sq. Typically, the sheet resistance of a conductive network formedby metal nanostructures is in the ranges of from 10 ohms/sq to 1000ohms/sq, from 100 ohms/sq to 750 ohms/sq, 50 ohms/sq to 200 ohms/sq,from 100 ohms/sq to 500 ohms/sq, or from 100 ohms/sq to 250 ohms/sq, or10 ohms/sq to 200 ohms/sq, from 10 ohms/sq to 50 ohms/sq, or from 1ohms/sq to 10 ohms/sq. For the opto-electrical devices described herein,the sheet resistance is typically less than 20 ohms/square, or less than15 ohms/square, or less than 10 ohms/square.

Optically, the nanostructure-based transparent conductors have highlight transmission in the visible region (400 nm-700 nm). Typically, thetransparent conductor is considered optically clear when the lighttransmission is more than 70%, or more typically more than 85% in thevisible region. More preferably, the light transmission is more than90%, more than 93%, or more than 95%. As used herein, unless specifiedotherwise, a conductive film is optically transparent (e.g., more than70% in transmission). Thus, transparent conductor, transparentconductive film, layer or coating, conductive film, layer or coating,and transparent electrode are used interchangeably.

Haze is an index of optical clarity. Haze results from light-scatteringand reflection/refraction due to both bulk and surface roughnesseffects. For certain opto-electrical devices such as PV cells and OLEDlighting applications, high-haze transparent conductors may bepreferred. These transparent conductors typically have haze values ofmore than 10%, more typically more than 15%, or more typically, in therange of 20%-50%. See Published U.S. Patent Application No.2011/0163403. For other devices such as OLED for display applications,low-haze is preferred. Additional details for adjusting or reducing hazecan be found, for example, Published U.S. Patent Application No.2009/0321113. These published U.S. patent applications are co-pendingapplications assigned to Cambrios Technologies Inc., the assignee of thepresent disclosure.

Unless otherwise specified, the haze value of a give transparentconductor described and claimed herein is measured photo-optically inaccordance with ASTM D 1003-07, “Standard Test Method for Haze andLuminous Transmittance of Transparent Plastics.”

Matrix

“Matrix” refers to a solid-state material into which the metal nanowiresare dispersed or embedded. Portions of the nanowires may protrude fromthe matrix to enable surface access to the conductive network. Thematrix is a host for the metal nanowires and provides a physical form ofthe conductive layer. The matrix protects the metal nanowires fromadverse environmental factors, such as corrosion and abrasion. Inparticular, the matrix significantly lowers the permeability ofcorrosive elements in the environment, such as moisture, trace amount ofacids, oxygen, sulfur and the like.

In addition, the matrix offers favorable physical and mechanicalproperties to the conductive layer. For example, it can provide adhesionto the substrate. Furthermore, unlike the fragile metal oxide films,polymeric or organic matrices embedded with metal nanowires can berobust and flexible. As will be discussed in more detail herein,flexible matrices make it possible to fabricate transparent conductorsin a low-cost, high throughput process.

Moreover, the optical properties of the conductive layer can be tailoredby selecting an appropriate matrix material. For example, reflectionloss and unwanted glare can be effectively reduced by using a matrix ofa desirable refractive index, composition and thickness.

Typically, the matrix is an optically clear material. A material isconsidered optically clear if the light transmission of the material isat least 80% in the visible region (400 nm-700 nm). Unless specifiedotherwise, all the layers (including the substrate) in a transparentconductor described herein are preferably optically clear. The opticalclarity of the matrix is typically determined by a multitude of factors,including without limitation: the refractive index (RI), thickness,consistency of RI throughout the thickness, surface (includinginterface) reflection, and haze (a scattering loss caused by surfaceroughness and/or embedded particles).

In certain embodiments, the matrix is a binder, i.e., the matrix isinitially dispersed in the ink composition with the nanostructures. Inthese embodiments, the terms “matrix” and “binder” are interchangeable.Following deposition on a substrate, the matrix cures as the volatilecomponents of the ink composition are removed or evaporate.

In other embodiments, the matrix is formed after the ink composition isdeposited on a substrate. In this regard, in addition to providing amedium for suspending the nanostructures, the matrix may also form aprotective layer or overcoat overlying the nanostructures. U.S. Pat. No.8,049,333, which is incorporated herein by reference in its entirety,provides detailed description of forming matrix.

In certain embodiments, the matrix is about 10 nm to 5 μm thick, about20 nm to 1 μm thick, or about 50 nm to 200 nm thick. In otherembodiments, the matrix has a refractive index of about 1.3 to 2.5, orabout 1.35 to 1.8.

In certain embodiments, the matrix is a polymer, which is also referredto as a polymeric matrix. Optically clear polymers are known in the art.Preferably, the polymer is crosslinklable or reflowable (e.g., flowableafter curing upon heating). Examples of suitable polymeric matricesinclude, but are not limited to: polyacrylics such as polymethacrylates(preferably, poly(methyl methacrylate)), polyacrylates andpolyacrylonitriles, polyvinyl alcohols, polyesters (e.g., polyethyleneterephthalate (PET), polyester naphthalate, and polycarbonates),polymers with a high degree of aromaticity such as phenolics orcresol-formaldehyde (Novolacs®), polystyrenes, polyvinyltoluene,polyvinylxylene, polyimides, polyamides, polyamideimides,polyetherimides, polysulfides, polysulfones, polyphenylenes, andpolyphenyl ethers, polyurethane (PU), epoxy, polyolefins (e.g.polypropylene, polymethylpentene, and cyclic olefins),acrylonitrile-butadiene-styrene copolymer (ABS), cellulosics, siliconesand other silicon-containing polymers (e.g. polysilsesquioxanes andpolysilanes), polyvinylchloride (PVC), polyacetates, polynorbornenes,synthetic rubbers (e.g. EPR, SBR, EPDM), and fluoropolymers (e.g.,polyvinylidene fluoride, polytetrafluoroethylene (TFE) orpolyhexafluoropropylene), copolymers of fluoro-olefin and hydrocarbonolefin (e.g., Lumiflon®), and amorphous fluorocarbon polymers orcopolymers (e.g., CYTOP® by Asahi Glass Co., or Teflon® AF by Du Pont).

In other embodiments, the matrix is an inorganic material. For example,a sol-gel matrix based on silica, mullite, alumina, SiC, MgO—Al₂O₃—SiO₂,Al2O₃—SiO₂, MgO—Al₂O₃—SiO₂—Li₂O or a mixture thereof can be used.

In certain embodiments, the matrix itself is conductive. For example,the matrix can be a conductive polymer. Conductive polymers are wellknown in the art, including without limitation:poly(3,4-ethylenedioxythiophene) (PEDOT), polyanilines, polythiophenes,and polydiacetylenes.

Anti-Reflective Layer

An anti-reflective layer can take the form of a Rayleigh's film based onthe principle of index matching, or an interference film based ondestructive interference.

Rayleigh's film is a thin film interposed between two layers that havedifferent indices of refraction, e.g., a substrate and an organic layer(of an OLED). The index of refraction of the anti-reflective layer is avalue selected between those of the substrate and the organic layer(i.e., “index matching”). The presence of the anti-reflective layermitigates the large difference of the indices of refraction of thesubstrate and the organic layer, thus reducing the internal reflectionat their respective interfaces.

In various embodiments, the anti-reflective layer may have an index ofrefraction in the range of 1.5-1.8, or in the range of 1.55-1.6, or inthe range of 1.6-1.65, or in the ranges of 1.65-1.7, or 1.7-1.75, or1.75-1.8.

The anti-reflective layer is typically optically transparent and has athickness between 200 nm to 2 microns.

In a preferred embodiment, the anti-reflective layer is a polyimidelayer. Typically, polyimides (regardless of the specific chemicalmoieties thereof) have indices of refraction of about 1.6, which valueis between those of a typical substrate (e.g., glass) and the organicstack, which tends to have a much higher index of refraction than thatof the substrate.

The anti-reflective layer can be typically deposited on a substrateaccording to known methods in the art, which includes spin-coating, slotdie coating or gravure coating, etc.

As an alternative to an index matching Rayleigh's film, a multi-layerinterference film may also be used. Such an interference film typicallycomprises alternating layers of low refractive index material and highrefractive index material, the thickness of which can be selected andoptimized depending on the wavelength to be transmitted.

Light-Emitting Layer

The light-emitting layer is a component of the organic stack in theOLED, according to one embodiment. The light-emitting layer can be anorganic material capable of emitting light when a current is passedbetween the anode (30) and the cathode. Preferably, the light-emittinglayer contains a phosphorescent emissive material, although fluorescentemissive materials may also be used. Phosphorescent materials arepreferred because of the higher luminescent efficiencies associated withsuch materials. The light-emitting layer may also comprise a hostmaterial capable of transporting electrons and/or holes, doped with anemissive material that may trap electrons, holes, and/or excitons, suchthat excitons relax from the emissive material via a photoemissivemechanism. The light-emitting layer may comprise a single material or amaterial that combines transport and emissive properties.

Photo-Active Layer

The photo-active layer is also a type of organic stack, which is thelight-absorbing component of a PV cell that converts light directly intoelectricity.

The photo-active layer may be one or more of the followingsemiconductive materials: monocrystalline silicon, polycrystallinesilicon, amorphous silicon, cadmium telluride, and copper indiumselenide/sulfide. Other suitable materials include thin-film layers oforganic dyes, and/or organic polymers. Alternatively, nanocrystals orquantum dots (electron-confined nanoparticles) may be used as thelight-absorbing material.

The photo-active layer can be a single layer, or more typically, inmultiple physical configurations to take advantage of different lightabsorption and charge separation mechanisms.

Scattering Centers

As used herein, scattering centers are formed by light-scatteringmaterial, which is an inert material that causes light scattering. Thelight-scattering material includes, for example, particulate scatteringmedia or scattering-promoting agents (e.g., precursors).

In certain embodiments, the light-scattering material is in the form ofparticles, also referred to as “light-scattering particles,” which canbe directly incorporated into a coating solution of polyimide. Followingcoating of the polyimide solution on the substrate, the light-scatteringparticles are distributed randomly in the polyimide film.

The light-scattering particles are preferably micro-sized particles, ormore preferably nano-sized particles. Typically, the particle sizes arein the range of 1 nm to several microns; preferably in the range of 10nm-800 nm, 10 nm-600 nm, 10 nm-400 nm, or 10 nm-200 nm. More typically,the particle sizes are in the range of 1 nm-100 nm.

The light-scattering particles may be an inorganic material, which maybe conductive, semiconductive, or non-conductive. Typically, the indexof refraction of the light scattering material should be high (e.g.,more than 1.6, or more typically, more than 1.7, or more typically,about 1.8). Examples of suitable light-scattering particles include,without limitation, SiO_(x), AlO_(x), InO_(x), SnO_(x), ZnO_(x),Al-doped ZnO_(x) (AZO), indium tin oxide (ITO), Sb-doped SnO_(x) (ATO),TiO_(x), SiC, fluorine-doped SnO_(x) (FTO), and the like. Examples ofhigher refractive index particles include TiO_(x), AlO_(x), and ZnO_(x).Examples of conductive particles include ITO, AZO, ATO, and the like.Different oxidation ratios (stoichiometries and hence doping levels) maybe used, particularly with respect to systems that include three or moreelements (e.g., AZO, ATO, ITO). In particular and in preferredembodiments, such materials, compositions and doping levels may be usedfor the scattering additives and also act as a suitable buffer andinterface layer between the conductive nanostructure network and anadjacent semiconductor (e.g., a-Si, um-Si layer in a PV stack). Forexample, without limitation, AdNano® ZnO 20 and AdNano® Z805nanoparticles and AdNano® ZnO deionized water-based dispersion can beused in this way.

Additional description of the light-scattering materials can be found inPublished U.S. Patent Application No. 2011/0163403, which isincorporated herein by reference in its entirety.

Substrate

Any substrate suitable for conventional OLED is also suitable for thevarious embodiments of the present disclosure. Examples of rigidsubstrates include glass, polycarbonates, acrylics, and the like.

Examples of flexible substrates include, but are not limited to:polyesters (e.g., polyethylene terephthalate (PET), polyesternaphthalate, and polycarbonate), polyolefins (e.g., linear, branched,and cyclic polyolefins), polyvinyls (e.g., polyvinyl chloride,polyvinylidene chloride, polyvinyl acetals, polystyrene, polyacrylates,and the like), cellulose ester bases (e.g., cellulose triacetate, andcellulose acetate), polysulphones such as polyethersulphone, polyimides,silicones, and other conventional polymeric films.

EXAMPLES Example 1 Synthesis of Silver Nanowires

Silver nanowires were synthesized by the reduction of silver nitratedissolved in ethylene glycol in the presence of poly(vinyl pyrrolidone)(PVP) following the “polyol” method described in, e.g., Y. Sun, B.Gates, B. Mayers, & Y. Xia, “Crystalline silver nanowires by softsolution processing,” Nanoletters 2(2): 165-168, 2002. A modified polyolmethod, described in co-pending and co-owned U.S. patent applicationSer. No. 11/766,552, produces more uniform silver nanowires at higheryields than does the conventional “polyol” method. This application isincorporated by reference herein in its entirety. Resulting nanowiresprimarily had lengths from about 13 μm to about 17 μm and diameters fromabout 34 nm to about 44 nm.

Example 2 Standard Preparation of Coating Composition of ConductiveNanostructures

A typical coating composition for depositing metal nanowires comprises,by weight, from 0.0025% to 0.1% surfactant (e.g., a preferred range isfrom 0.0025% to 0.05% for ZONYL® FSO-100), from 0.02% to 4% viscositymodifier (e.g., a preferred range is 0.02% to 0.5% for hydroxypropylmethylcellulose (HPMC), from 94.5% to 99.0% solvent and from 0.05% to1.4% metal nanowires.

The coating composition can be prepared based on a desired concentrationof the nanowires, which is an index of the loading density of the finalconductive film formed on the substrate.

The coating composition can be deposited on a substrate according to,for example, the methods described in co-pending U.S. patent applicationSer. No. 11/504,822.

As understood by one skilled in the art, other deposition techniques canbe employed, e.g., sedimentation flow metered by a narrow channel, dieflow, flow on an incline, slit coating, gravure coating, microgravurecoating, bead coating, dip coating, slot die coating, and the like.Printing techniques can also be used to directly print an inkcomposition onto a substrate with or without a pattern. For example,inkjet, flexoprinting and screen printing can be employed. It is furtherunderstood that the viscosity and shear behavior of the fluid as well asthe interactions between the nanowires may affect the distribution andinterconnectivity of the nanowires deposited.

A sample conductive nanostructure dispersion was prepared that comprisedsilver nanowires as fabricated in Example 1 dispersed, a surfactant(e.g., Triton), and a viscosity modifier (e.g., low molecular-weightHPMC) and water. The final dispersion included about 0.4% silver and0.4% HPMC (by weight). This dispersion can be used (neat or diluted) incombination with a light-scattering material (e.g., in a particulateform) directly to form a coating solution. Alternatively, the dispersioncan be combined with a dispersion of a light-scattering material to forma coating solution.

Example 3

A polyimide coating solution (e.g., SUNEVER Polyimide (type 0821)) wasfirst deposited on a substrate, spun on at 1500 rpm, followed by dryingat 90° C., and curing for 30 min at 200° C. The haze and transmission ofthe resulting sample were 0.1% and 92.1%, respectively. The filmthickness was measured at 1.2 microns.

Example 4

Silver nanowires were deposited on an anti-reflective layer, e.g., apolyimide film, to form a conductive film. A standard nanowiresuspension was first prepared according to Example 2 (0.4% AgNW, 0.4%LMw HPMC, 250 ppm Triton X). The coating solution was spun on thepolyimide film at 1000 rpm, followed by drying for 90 seconds at 50° C.and annealing for 90 seconds at 140° C. The resulting sheet resistanceis 9 ohms/sq with 87.5% in transmission and 3.9% in haze.

Compared to a device without the polyimide layer, i.e., the nanowireswere deposited directly on glass, the optical data as well as the sheetresistance were substantially identical. The anti-reflective layer doesnot impact the optical and electrical performance of the nanostructurelayer.

The various embodiments described above can be combined to providefurther embodiments. All of the U.S. patents, U.S. patent applicationpublications, U.S. patent application, foreign patents, foreign patentapplication and non-patent publications referred to in thisspecification and/or listed in the Application Data Sheet areincorporated herein by reference, in their entirety. Aspects of theembodiments can be modified, if necessary to employ concepts of thevarious patents, application and publications to provide yet furtherembodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

The invention claimed is:
 1. An optical stack comprising: a firstelectrode; an organic stack underlying the first electrode; ananostructure layer underlying the organic stack, the nanostructurelayer comprises a plurality of metal nanostructures; a high-index layerunderlying the nanostructure layer; and a substrate underlying thehigh-index layer, wherein the high-index layer has the same or a higherrefractive index than the organic stack, and wherein the nanostructurelayer forms a second electrode, and comprises a high-index matrix,wherein the high-index matrix has the same or a higher refractive indexthan the organic stack.
 2. The optical stack of claim 1 wherein, whenwaveguided light within the optical stack is represented by an energydensity distribution curve, the nanostructure layer is positioned suchthat it overlaps with at least 10% of the energy density distributioncurve.
 3. An optical stack comprising: a first electrode; an organicstack underlying the first electrode; a nanostructure layer underlyingthe organic stack, the nanostructure layer comprises a plurality ofmetal nanostructures; a high-index layer underlying the nanostructurelayer; and a substrate underlying the high-index layer, wherein thehigh-index layer has the same or a higher refractive index than theorganic stack, and wherein the nanostructure layer forms a secondelectrode, and comprises a low-index matrix, wherein the low-indexmatrix has a lower refractive index than the organic stack.
 4. Atop-emitting OLED comprising: a substrate; a first electrode disposed onthe substrate; an organic stack disposed on the first electrode; and ananostructure layer including a plurality of nanostructures overlyingthe organic stack, wherein the nanostructure layer is a secondelectrode, and wherein the nanostructure layer further comprises amatrix, in which the plurality of nanostructures are embedded.
 5. AnOLED comprising: a substrate; a bottom electrode disposed on thesubstrate; an organic stack disposed on the bottom electrode; and ametal film disposed on the organic stack, wherein the metal film has anouter surface and contacts the organic stack by a metal/organicinterface, and wherein a first plurality of nanostructures are disposedon the metal/organic interface, and a second plurality of nanostructuresare disposed on the outer surface.
 6. The OLED of claim 5 wherein thesecond plurality of nanostructures are embedded in a matrix disposed onthe outer surface.
 7. An OLED comprising: a transparent substrate; atransparent bottom electrode disposed on the transparent substrate; anorganic stack disposed on the transparent bottom electrode; and a metalfilm disposed on the organic stack, wherein the metal film contacts theorganic stack by a metal/organic interface and has an outer surface, andwherein a plurality of nanostructures are disposed on the outer surface,the plurality of nanostructures being embedded in a matrix.
 8. Anorganic light-emitting diode (OLED) comprising: a substrate having a topsurface and a bottom surface, the top surface being an interface betweenthe substrate and air; an anti-reflective layer contacting the bottomsurface of the substrate; a first electrode deposited on theanti-reflective layer, the first electrode comprising a plurality ofconductive nanostructures; an organic stack deposited on the firstelectrode, the organic stack comprising an organic light-emittingmaterial, a charge injection layer and a hole injection layer; and asecond electrode deposited on the organic stack.
 9. The OLED of claim 8wherein the substrate has a first refractive index, and the organicstack has a second refractive index, and the anti-reflective layer has athird refractive index, and wherein the third refractive index is largerthan the first refractive index and less than the second refractiveindex.
 10. The OLED of claim 8 wherein the anti-reflective layer has anindex of reflection in the range of 1.5-1.8.
 11. The OLED of claim 8wherein the anti-reflective layer is a polyimide layer.
 12. The OLED ofclaim 8 wherein the first electrode is an anode and the second electrodeis a cathode.
 13. The OLED of claim 8 wherein the substrate is glass.14. The OLED of claim 8 wherein the anti-reflective layer furthercomprises a plurality of light-scattering particles.
 15. The OLED ofclaim 14 wherein the light-scattering particles are SiO_(x), AlO_(x),InO_(x), SnO_(x), ZnO_(x), Al-doped ZnO_(x) (AZO), indium tin oxide(ITO), Sb-doped SnO_(x) (ATO), TiO_(x), SiC, or fluorine-doped SnO_(x)(FTO).
 16. An organic light-emitting diode (OLED) comprising: a firstelectrode; an organic stack underlying the first electrode, the organicstack comprising an organic light-emitting material, a charge injectionlayer, and a hole injection layer; a second electrode underlying theorganic stack having a plurality of interconnecting conductivenanostructures, wherein the second electrode further comprises aninsulating binder in which the conductive nanostructures are embedded,and wherein the insulating binder has a third refractive index; a firsthigh-index layer underlying the second electrode; and a substrateunderlying the high-index layer, wherein light generated in the organicstack exits from the substrate; and wherein the organic stack has afirst refractive index, and the high-index layer has a second refractiveindex, and wherein the second refractive index is the same or higherthan the first refractive index.
 17. The OLED of claim 16 wherein thesecond refractive index is higher than the third refractive index. 18.The OLED of claim 16 wherein the second refractive index is the same asthe third refractive index, and wherein the binder and the high-indexlayer are of a same material.
 19. The OLED of claim 18 furthercomprising a second high-index layer underlying the first high-indexlayer.
 20. The OLED of claim 19 wherein the first high-index layer andthe second high-index layer further include a plurality of lightscattering particles.
 21. The OLED of claim 19 wherein the firsthigh-index layer and the second high-index layer are polyimide.
 22. Anoptical stack comprising: a first electrode; an organic stack underlyingthe first electrode; a nanostructure layer underlying the organic stack,the nanostructure layer comprises a plurality of metal nanostructures; ahigh-index layer underlying the nanostructure layer; and a substrateunderlying the high-index layer, wherein the high-index layer has thesame or a higher refractive index than the organic stack, and whereinthe nanostructure layer forms a second electrode, wherein, whenwaveguided light within the optical stack is represented by an energydensity distribution curve, the nanostructure layer is positioned suchthat it overlaps with at least 10% of the energy density distributioncurve.
 23. The optical stack of claim 22 wherein the nanostructure layercomprises a low-index matrix, wherein the low-index matrix has a lowerrefractive index than the organic stack.